In recent years push-button telephones have come into widespread use. Such telephones operate by generating a selected pair of tones for each key. Thus a tone generator must be provided within the telephone for generating each of the fundamental tones. Heretofore the most widely used approach in generating the tones has been to use LC circuits together with discrete transistors. But this is an expensive approach for generating the eight tones required in a push-button telephone.
It has been proposed to utilize integrated circuits for generating the dial tones and such circuits have been marketed. The standards for tone generators are laid out for the Bell system in Bell Publication 47001 entitled "Electrical Characteristics of Bell System's Network Facilities at the Interface with Voiceband, Ancillary and Data Equipment." The Rural Electrification Administration (REA) standards are given in publication 345-74 and the Electronic Industry Association (EIA) standards are in the EIA publication SP 1286. In addition to meeting these specifications, customer requirements frequently require that tone circuits work at voltages as low as 2 volts. This requirement is a problem due to the threshold voltages of MOS (metal oxide semiconductor) transistors.
A further problem is presented by telephone specifications, especially European, in regard to total harmonic distortion. The techniques for generating tones using MOS technology have heretofore utilized a relatively small number of step voltages for each cycle of the sine wave. When a tone signal is generated in this manner there is substantial harmonic distortion at relatively low frequencies and this distortion frequently exceeds European specification limits. Further, it is substantially more difficult to farbricate on-chip filters for low frequencies rather than high frequencies.
Therefore, there exists a need for a design of a circuit for fabrication in MOS technology which can produce the tone signals required in a DTMF (dual-tone-multi-frequency) telephone system in which the integrated circuit can successfully work at a voltage as low as 2.0 volts and at the same time generate no more distortion within the frequency limits allowed under the various telephone system standards and customer requirements.